The present invention relates to a controlling circuit and method in a CDMA (Code Division Multiple Access) system in which, during an operation at the maximum transmission power, the transmission power is controlled.
Conventionally, as a technique of this kind, a power controlling circuit for a transmitter is disclosed in Unexamined Japanese Patent Publication (Kokai) HEI8-46466. FIG. 2 shows an example of the configuration of the disclosed power controlling circuit for a transmitter. As shown in the figure, the power controlling circuit comprises a gain-variable amplifier 22, an RF (Radio Frequency) power amplifier 23, a directional coupler 24, a detection circuit 26, a comparator 28, a switch 29, and an adder 30.
The gain-variable amplifier 22, the RF power amplifier 23, the directional coupler 24, the detection circuit 26, the comparator 28, the switch 29, and the adder 30 are connected so as to constitute a saturation prevention control loop 40. When an output of the comparator 28 is "H" (high level), the switch 29 is in a conductive state, and, when the output is "L" (low level), the switch is in a nonconductive state. A system power control terminal 31, the adder 30, and the gain-variable amplifier 22 are connected so as to constitute a system power control loop 41.
Since the switch 29 is disposed in the saturation prevention control loop 40, the saturation prevention control loop 40 operates only when the level of the amplified signal is larger than a reference value. In a state other than the above, the system power control loop 41 appropriately operates on the basis of a system power control signal which is supplied through the system power control terminal 31.
In the case where transmission is conducted at the maximum transmission power by means of the system power control loop 41, when the output is caused to exceed the maximum output power by various reasons such as a temperature change, the switch 29 is turned on so that the system power control voltage is lowered. As a result, the RF power amplifier 23 is prevented from saturating and its linear characteristics are maintained.
In the prior art method described above, saturation of the RF power amplifier 23 can be prevented from occurring by the saturation prevention control loop 40. Even though the maximum transmission power is preset by the system power control loop 41, however, a predetermined power may fail to be transmitted because of various reasons such as a temperature change. In such a case, correction is not conducted and hence there arises a problem in that the communication range is narrowed.